Coupled Surface Modes Research Articles

Single vector mode transmission in hollow-core photonic bandgap fiber based on surface mode coupling effect

Single vector mode transmission in hollow-core photonic bandgap fiber based on surface mode coupling effect

Spectral tuning of Bloch Surface Wave resonances by light-controlled optical anisotropy

Abstract Fostered by the recent advancements in photonic technologies, the need for all-optical dynamic control on complex photonic elements is emerging as more and more relevant, especially in integrated photonics and metasurface-based flat-optics. In this framework, optically-induced anisotropy has been proposed as powerful mean enabling tuning functionalities in several planar architectures. Here, we design and fabricate an anisotropic two-dimensional bull’s eye cavity inscribed within an optically-active polymeric film spun on a one-dimensional photonic crystal sustaining Bloch surface waves (BSW). Thanks to the cavity morphology, two surface resonant modes with substantially orthogonal polarizations can be coupled within the cavity from free-space illumination. We demonstrate that a dynamic control on the resonant mode energies can be easily operated by modulating the orientation of the optically-induced birefringence on the surface, via a polarized external laser beam. Overall, reversible blue- and red-shifts of the resonant BSWs are observed within a spectral range of about 2 nm, with a moderate laser power illumination. The polymeric structure is constituted by a novel blend of an azopolymer and a thermally-sensitive resist, which allows a precise patterning via thermal scanning probe lithography, while providing a significant structural integrity against photo-fluidization or mass-flow effects commonly occurring in irradiated azopolymers. The proposed approach based on tailored birefringence opens up new pathways to finely control the optical coupling of localized surface modes to/from free-space radiation, particularly in hybrid organic–inorganic devices.

Near-perfect (>99%) dual-band absorption in the visible using ultrathin semiconducting gratings.

Electromagnetic perfect absorption entails impedance-matching between two adjacent media, which is often achieved through the excitation of photonic/plasmonic resonances in structures such as metamaterials. Recently, super absorption was achieved using a simple bi-layer configuration consisting of ultrathin lossy films. These structures have drawn rising interest due to the structural simplicity and mechanical stability; however, the relatively broadband absorption and weak angular dependence can limit its versatility in many technologies. In this work, we describe an alternative structure based on an ultrathin semiconducting (Ge) grating that features a dual-band near-perfect resonant absorption (99.4%) in the visible regime. An angular-insensitive resonance is attributed to strong interference inside the ultrathin grating layer, akin to the resonance obtained with a single ultrathin planar film, while an angular-sensitive resonance shows a much narrower linewidth and results from the diffraction-induced surface mode coupling. With an appropriately designed grating period and thickness, strong coherent coupling between the two modes can give rise to an avoided-crossing in the absorption spectra. Further, the angular-insensitive resonance can be tuned separately from the angularly sensitive one, yielding a single narrow-banded absorption in the visible regime and a broadband absorption resonance that is pushed into the near-infrared (NIR). Our design creates new opportunities for ultra-thin and ultra-compact photonic devices for application in technologies including image sensing, structural color-filtering and coherent thermal light-emission.

Coupling of Photonic Crystal Surface Modes

Guiding and evanescent coupling properties of surface modes bound to the interfaces of two-dimensional photonic crystals in close proximity are numerically demonstrated. Interacting photonic crystals are composed of silicon pillars in air, where their outermost layers facing each other are annular. Surface modes are identified through supercell band structure computations, while their excitation by the electromagnetic waves through a perpendicular insertion waveguide is demonstrated using finite-difference time-domain simulations. Lifting the degeneracy between the surface modes as a consequence of bringing two identical photonic crystal surfaces to a sufficient distance results in evanescent coupling in a beating manner whose beat length linearly varies between 10 and 20 periods up to a frequency at which both surface modes travel with the same group velocity. The surface mode coupling phenomenon could be employed either to enhance sensitivity or to reduce device size in bio/chemical sensor applications since the effective travelling length of surface waves increases by about 3.5 times due to evanescent coupling.

Selective Properties of Mid-Infrared Tamm Phonon-Polaritons Emitter with Silicon Carbide-Based Structures.

Electromagnetic (EM) absorbers and emitters have attracted much interest because of their versatile applications. A photonic heterostructure composed of silicon carbide (SiC) layer/germanium (Ge) cavity/distributed Bragg reflector (DBR) has been proposed. Selective emission properties have been investigated through rigorous coupled wave analysis (RCWA) method. The results illustrate that Tamm phonon-polaritons can be excited, and the magnetic field is partially centralized at the junction of Ge cavity and SiC film, aimed to improve the interactions of photon–phonon. The absorptivity/emissivity of the structure can be better optimized by controlling the coupling of surface modes with the incident wave. Near-unity absorption can be achieved through optimizing the SiC grating/Ge cavity/distributed Bragg reflector (DBR) multilayer structure with geometrical parameters of ds = 0.75 μm, dg = 0.7 μm, d1 = 1.25 μm and d2 = 0.75 μm, respectively. Physical mechanism of selective emission characteristics is deliberated. In addition, the simulation results demonstrate that the emitter desensitizes to the incidence angle and polarization state in the mid-infrared (MIR) range. This research ameliorates the function of the selective emitters, which provides more efficient design for SiC-based systems.

Temperature-Dependent Group Delay of Photonic-Bandgap Hollow-Core Fiber Tuned by Surface-Mode Coupling.

Surface modes (SM) are highly spatially localized modes existing at the core-cladding interface of photonic-bandgap hollow-core fiber (PBG-HCF). When coupling with SM, the air modes (AM) in the core would suffer a higher confinement loss despite being spectrally within the cladding photonic bandgap, and would be highly dispersive around the avoided crossing (anti-crossing) wavelength. In this paper, we numerically explored how such avoided crossings can play an important role in the tuning of the temperature dependence of group delay of AM of PBG-HCF. At higher temperatures, both the thermo-optic effect and thermal expansion contribute to the redshift of avoided crossing wavelength, giving rise to a temperature dependence of the AM dispersion. Numerical simulations show that the redshift of avoided crossing can significantly tune the thermal coefficient of delay (TCD) of PBG-HCF from -400 ps/km/K to 400 ps/km/K, approximately -120 ppm/K to 120 ppm/K. In comparison with the known tuning mechanism by thermal-induced redshift of photonic bandgap [Fokoua et al., Optica 4, 659, 2017], the tuning of TCD by SM coupling presents a much broader tuning range and higher efficiency. Our finding may provide a new route to design PBG-HCF for propagation time sensitive applications.

Coupling of Upper Hybrid Surface Plasmon Modes in Magnetoplasma Waveguide Structure

Abstract: We investigate the spectra of high-frequency electrostatic surface electron plasmon oscillations propagating normal to a dc-magnetic field. These oscillations are supported by two identical magnetoplasma slabs separated by a vacuum slab. Propagation characteristics of surface magnetoplasma oscillations and their coupling are studied by simultaneously solving the homogeneous system of equations obtained by matching the electrostatic fields at the interfaces together with the warm plasma dielectric function of upper hybrid waves. We demonstrate the existence of two propagating magnetoplasma electrostatic surface modes (backward and forward modes). The backward mode emerges at frequency ω=ω_uh=√(ω_pe^2+ω_ce^2 ), where ω_pe and ω_ce are the electron plasma frequency and the electron cyclotron frequency, respectivily, and the forward propagating mode emerges at a lower frequency ω=ω_uh-ω_pe. The forward and backward surface modes become coupled and form a single mode at upper hybrid resonance quasi-static value ω=ω_uh/√2. Keywords: Upper hybrid modes, Plasma slab waveguide, Coupled plasmon surface modes.

Near-field radiative heat transfer enhancement in the thermophotovoltaic system using hyperbolic waveguides

As an energy conversion system, the near-field thermophotovoltaic (NTPV) device has attracted much attention. To improve the NTPV system output power, an intermediate modulator with the same plasmonic properties as thermal emitters can be intercalated into the two-body system. Surface modes are coupled in the vacuum gap between the modulator and the emitter. The modulator absorbs the enormous radiative energy produced by the coupling of surface modes and transfers it to the cell. However, due to the low penetration depth of surface modes and the loss of energy transmission in the intermediate slab, the direct heat exchange between the emitter and the cell in the three-body system is much smaller than that in the corresponding two-body system. In the present study, we design a hyperbolic metamaterial (HMM)-based three-body NTPV system. Ga-doped ZnO (GZO)/HfO2-multilayer-based HMMs serve as the emitter and the modulator, and GaSb works as the cell. The radiative heat flux from the emitter to the cell is enhanced by the weakly dissipating hyperbolic waveguide, which is attributed to the coupling of the vacuum-GZO surface plasmon polariton (SPP) and the SPPs at the GZO-HfO2 interfaces in the HMM modulator. Compared with the GZO slab emitter, the HMM emitter emits higher spectral heat flux located in the hyperbolic region. Different top layers of the HMM affect the coupling of surface modes. These findings provide a new way to enhance the three-body NTPV system output power.

Enhanced near-field radiation in both TE and TM waves through excitation of Mie resonance

Near-field radiative heat transfer between two bodies can exceed the far-field blackbody limitation predicted by Planck's law due to the evanescent waves tunneling or coupling of additional surface modes, which typically can occur only in TM modes for nonmagnetic materials. The Mie resonance may have the potential to enhance near-field radiation in both TM and TE modes according to the calculated results from effective medium theory. However, there is no exact solution to verify this to date. In this paper, we will give a rigorous numerical investigation of the role of Mie resonance in near-field radiative heat transfer. The framework of fluctuational electrodynamics that combines scattering matrix theory with the rigorous coupled wave analysis method is used to exactly compute the near-field radiative flux between Mie resonant dielectric cubic arrays. It shows that due to the excitation of Mie resonance, causing $\ensuremath{\varepsilon}/\ensuremath{\mu}$ near-pole effects, the radiative heat flux could be spectrally enhanced in TM/TE modes. The discrepancy between the effective medium theory and the exact method is also elucidated in detail. The findings will provide a way to control near-field thermal spectrum via all-dielectric metamaterials for emerging thermal technologies.

Thermal rectification and spin-spin coupling of nonreciprocal localized and surface modes

We study the rectification of near-field radiative heat transfer between two InSb nano-particles due to the presence of non-reciprocal surface modes in a nearby InSb sample when an external magnetic field is applied and its dependence on the magnetic field strength. We reveal the spin-spin coupling mechanism of the localized particle resonances and the surface mode resonances which is substantiated by the directional heat flux in the given setup. We discuss further the interplay of the frequency shift, the propagation length, and local density of states on the strength and directionality of the rectification as well as the non-reciprocal heating effect of the nanoparticles.

Enhancing near-field heat transfer between composite structures through strongly coupled surface modes

In this work, we study the near-field heat transfer between composite nanostructures. It is demonstrated that thermally excited surface plasmon polaritons, surface phonon polaritons, and hyperbolic phonon polaritons in such composite nanostructures significantly enhance the near-field heat transfer. To further analyze the underlying mechanisms, we calculate energy transmission coefficients and obtain the near-field dispersion relations. The dispersion relations of composite nanostructures are substantially different from those of isolated graphene, silicon carbide (SiC) films, and SiC nanowire arrays due to the strong coupling effects among surface polaritonic modes. We identify four pairs of strongly coupled polaritonic modes with considerable Rabi frequencies in graphene/SiC film composite structures that greatly broaden the spectral peak. We confirm that near-field strong coupling effects between surface plasmon polaritons and hyperbolic phonon polaritons in the in-plane Reststrahlen band are different from those in the out-of-plane Reststrahlen band due to the different types of hyperbolicity. In addition, we analyze the effective tunability of the near-field heat transfer of graphene/SiC nanowire arrays composite structures by adjusting the chemical potential of graphene, the height and volume filling factor of the SiC nanowire arrays. This work provides a method to manipulate the near-field heat transfer with the use of strongly coupled surface polaritonic modes.

Harvesting the Electromagnetic Energy Confined Close to a Hot Body

Abstract In the close vicinity of a hot body, at distances smaller than the thermal wavelength, a high electromagnetic energy density exists due to the presence of evanescent fields radiated by the partial charges in thermal motion around its surface. This energy density can surpass the energy density in vacuum by several orders of magnitude. By approaching a photovoltaic (PV) cell with a band gap in the infrared frequency range, this nonradiative energy can be transferred to it by photon tunnelling and surface mode coupling. Here we review the basic ideas and recent progress in near-field energy harvesting.

Coupled Topological Surface Modes in Gyrotropic Structures: Green's Function Analysis

At a transition in a wave-guiding structure, part of the incident energy is transmitted and part of the energy is reflected. When the waveguide has non-trivial topological properties, however, the transition may occur with no backscattering, and with unusual modal coupling/transformations. Within this context, we discuss the response of a nonreciprocal topological structure composed of two nearby interfaces between oppositely-biased gyrotropic media and an isotropic medium, which support unidirectional surface modes (topological modes). We provide an exact Green's function analysis of this structure, and we discuss how the topological surface modes are modified when the two interfaces are brought closer and eventually merged. We show that the resulting mode conversion is independent of the transition geometry.

Volume and surface mode coupling experiments in periodic surface structures for use in mm-THz high power radiation sources

Planar Periodic Surface Lattice (PSL) structures of different configurations have been designed, fabricated and measured in the 140-220 GHz frequency band. Surface mode resonances are observed in ‘mesh’ PSL structures. We establish that, when mounted on suitable metal-backed dielectric substrates, PSLs exhibit ‘mode-locked’ coherent cavity eigenmodes formed from coupled volume and surface modes. The ‘proof-of-principle’ coupling of volume and surface modes and concept of mode selection in a large cavity, which can lead to the innovation of high power mm-THz radiation sources, is demonstrated. Evidence of coupled eigenmode formation in a 0.64 mm planar PSL measured at 325-500 GHz is presented, verifying the scalability of this work. Excellent agreement between numerical modelling and experiment is reported.

Surface modes with controlled axisymmetry triggered by bubble coalescence in a high-amplitude acoustic field

When two bubbles encounter each other in a moderate ultrasound field they coalesce into a single bubble that shows purely spherical oscillations. For a sufficiently large acoustic field, this coalescence can lead to sustained nonspherical oscillations of the resulting bubble. We experimentally capture the time-resolved dynamics of the coalesced bubble, starting with the moment of film rupture. This allows the transient and steady-state regimes of the oscillating bubble to be studied. The amplitude dynamics for both of these regimes are successfully compared to numerical modeling, taking into account the coupling of volume and surface modes. Initial conditions for each surface mode are taken from the Legendre polynomial projection of the experimentally obtained bubble shape immediately after film rupture. We also observe that the symmetry axis of the zonal spherical harmonics is defined by the coalescence. The axis is identical to the rectilinear translational motion of the two approaching bubbles before impact. This high-amplitude coalescence technique provides a unique opportunity to study axisymmetric and sustained nonspherical bubble oscillations under controlled initial conditions.

Narrow-core hollow optical waveguide with nanostructured SOI as ultra-low loss platform for efficient photodetection

A nanostructured hollow optical waveguide based on high-index contrast grating (HCG) embedded SOI is proposed. An ultra-low propagation loss of 1.22 dB/m even at narrow, 1- $$\upmu $$ m thick, air-core is reported. A high-performance photodetection is realized by the introduction of hollow core in form of intrinsic region in the photodetection (PIN) layer within HCG-assisted narrow-core waveguide. A sufficiently high responsivity of 0.8 A/W and quantum efficiency of 64% are obtained at 1550-nm which is possible because of the presence of surface modes within HCG which get coupled in the photodetection layer leading to a strong optical confinement in that layer. High reflectivity, small penetration length and coupling of lateral surface modes in HCG make it possible to offer improved waveguiding and hence photodetection.

Optical Properties of Transferred Graphene Nanostructures

This talk reports on recent progress in optical characterization of transferred graphene samples. After moving graphene, typically grown by CVD on Cu substrates, onto different substrates, not only its mechanical properties and continuity can be altered. Strain and unintentional doping could emerge that are resolvable by means of hyperspectral microscopy and statistical post-processing of the hyperspectral data. More standard optical techniques, such as transmission or reflection, can be also applied to investigate such samples. Simple optical models were developed to quantitatively interpret the optical data. Coupling of graphene surface modes to the polaritons of the substrate is another phenomenon deserving attention. Hybridization of the 2D modes by graphene and polaritons produces a tangled response, qualitatively different in different frequency ranges of the probe.

Enhancement of quantum friction via coupling of surface phonon polariton and graphene plasmons

In our daily life, frictions are very common when two bodies in direct contact relatively move. However, when two bodies are separated by a finite distance, due to the quantum fluctuations inside the bodies, they may still experience a friction when they relatively move. Such a phenomenon is often called quantum friction, which has been studied for more than a decade. It has shown in previous studies that the surface modes, such as surface phonon polaritions (SPhPs) or surface plasmon polaritions (SPPs) have significant contribution to enhancing the quantum friction. However, to the best of our knowledge, the contribution of coupling from SPhPs and SPPs to quantum friction is still unknown. Here, we report a detailed study on the quantum frictions between two graphene sheets with the silicon carbide (SiC) substrates. For comparison, the quantum frictions between two other samples, i.e., SiC/SiC and graphene/graphene are also studied. As indicated in previous studies, SPhPs and SPPs, supported by SiC and graphene, respectively, can couple together in special frequency ranges. The coupling of SPhPs and SPPs can be tuned by varying the chemical potential of graphene. The coupling modes shift toward higher frequency as the chemical potential increases. Firstly, we analyze qualitatively the effects of coupled surface modes on quantum friction with the help of dispersion relation. Secondly, we calculate the quantum friction coefficients numerically for the three different samples. We find that due to the coupling of SPhPs and SPPs, the quantum friction between graphene sheets with SiC substrates is larger than that between the SiC or monolayer graphene sheets. We demonstrate that the coupling of SPhPs and SPPs can be modulated by chemical potential of graphene; therefore, the relationship between quantum friction coefficient and chemical potential is also studied. We observe that with the increase of chemical potential, quantum friction coefficient follows a non-monotonic trend, i.e., it first increases to its maximum value then decreases. We believe that our studies are not only helpful in understanding the micro mechanisms of friction, but also meaningful in the fabrications of micro- and nano-electromechanical systems.

Phononic crystal surface mode coupling and its use in acoustic Doppler velocimetry

It is numerically shown that surface modes of two-dimensional phononic crystals, which are Bloch modes bound to the interface between the phononic crystal and the surrounding host, can couple back and forth between the surfaces in a length scale determined by the separation of two surfaces and frequency. Supercell band structure computations through the finite-element method reveal that the surface band of an isolated surface splits into two bands which support either symmetric or antisymmetric hybrid modes. When the surface separation is 3.5 times the lattice constant, a coupling length varying between 30 and 48 periods can be obtained which first increases linearly with frequency and, then, decreases rapidly. In the linear regime, variation of coupling length can be used as a means of measuring speeds of objects on the order of 0.1m/s by incorporating the Doppler shift. Speed sensitivity can be improved by increasing surface separation at the cost of larger device sizes.

Epsilon-Near-Zero Modes for Tailored Light-Matter Interaction

Engineering collective excitations such as plasmons and polaritons provides access to exotic optical properties. This study reports semiconductor nanostructures with coupled surface modes that interact with multiple resonant dipoles, yielding double and triple polariton branches in the system's response. This produces useful and tunable properties with the flexibility to enable low-voltage tunable filters, light-emitting diodes, and efficient nonlinear composite materials.